1. Field of Invention
The present invention relates to projection devices and particularly to light sources and the preparation of light used in projection devices. The invention is yet more particularly related to projection devices using mercury short arc lamps as light sources for the projection devices.
2. Discussion of Background
The components of an LCOS based video projector 100 are explained by example of a light engine with reference to FIG. 1. As shown, white light 110 is generated by a light source 105. The light is collected, homogenized, polarized, and formed into the proper shape and otherwise processed by optics (not all shown for clarity). The light then enters a prism assembly 150 where it is broken into red, green and blue polarized light beams. A set of reflective microdisplays 152A, 152B, and 152C are provided and positioned to correspond to each of the polarized light beams (the prism assembly 150 with the attached microdisplays is called a kernel). The beams then follow different paths within the prism assembly 150 such that each beam is directed to a specific reflective microdisplay. The microdisplay that interacts with (reflects) the green beam modulates the green content of a full color video image. Similarly, the red and blue contents of the full color image are modulated by corresponding “red” and “blue” microdisplays. The prism assembly 150 then recombines the modulated beams into a modulated white light beam 160 that contains the full color video image. The resultant modulated white light beam 160 then exits the prism assembly 150 and enters a projection lens 165. Finally, the image-containing beam (white light beam 160 has been modulated and now contains the full color image) is projected onto a screen 170.
The light source 105 is, for example, a mercury short arc lamp. A mercury short arc lamp is the light source of choice in the vast majority of microdisplay based light engines of the type used in projection High Definition Television (HDTV). This includes light engines based on High Temperature Polysilicon (HTPS) microdisplays, Digital Light Processing (DLP) microdisplays and Liquid crystal on Silicon (LCOS) microdisplays. One unfavorable characteristic of the short arc lamp is that the light level tends to fluctuate. The light level variation can be of such a magnitude and frequency to be characterized as flicker. Flicker in the lamp is manifested as an unacceptable visible artifact in the projected video image. The predominant source of flicker can rightfully be attributed to instabilities in the arc.
Manufacturers of short arc lamps (such as Philips, the producer of the industry leading Ultra High Pressure (UHP) type lamp) have tried to minimize flicker. One technique is to pulse the current to the lamp. This technique is found to lessen the random component of the variation but, in itself, causes a predictable and periodic variation in light level. In practice, the periodic variation in light output is approximately canceled by synchronizing the lamp pulse to the drive of the microdisplay.